1. Field of the Invention
The present invention relates to a digital transmission test signal generating circuit suitable for checking digital signal transmission systems.
2. Description of the Prior Art
The SMPTE (Society of Motion Picture and Television Engineers, Inc.) of the United States is expected to adopt a scrambled NRZI (no-return-to-zero inverted code) method as its standard method for interfacing digital video/audio data primarily on an intra-office basis. This method involves having a transmitting side scramble 10-bit input data by performing a modulo-2 division thereon using a polynomial: EQU G(x)=(x+1)(x.sup.9 +x.sup.4 +1)
and having a receiving side descramble the data by performing a module-2 multiplication thereon using the same polynomial.
One disadvantage of this method is that its automatic gain control (AGC) circuit for automatic compensation of cable-induced deterioration can malfunction due to the presence of DC components generated upon input of a specific combination of video/audio data.
For example, suppose that 10-bit hexadecimal data "300" and "198" are supplied alternately, as shown in FIG. 5 (A). In this case, DC components are generated in the scrambled data. This is the worst state for the AGC circuit to operate in, as depicted in FIG. 5 (B).
Another disadvantage of the above method is that a specific combination of input data may suppress for an extended period of time the signal transition (i.e., from "1" to "0" or from "0" to "1") that would normally occur at substantially constant intervals.
For example, suppose that 10-bit hexadecimal data "110" and "200" are supplied alternately, as shown in FIG. 6 (A). In that case, signal transition only occurs every 20 clock pulses in the scrambled data. This is the worst state for the phase-locked loop (PLL) circuit to operate in for clock reproduction, as illustrated in FIG. 6 (B).
Statistically, the worst-case states described above almost never occur during ordinary signal transmissions. But there is no absolute guarantee that they will be completely eliminated.
In addition to the above cases discussed by the SMPTE, there also exist worst-state signals for the digital signal transmission system to operate with if the scrambling method remains the same regardless of the contents of the signals transmitted.
Since the incidence of such worst-case signals is not appreciably high, it has been conventionally difficult to check digital signal transmission systems (i.e., newly manufactured or readjusted equipment) for their performance with worst-case signals while such systems are allowed to operate under ordinary operating conditions.